The fabrication of ceramic, high-temperature process equipment, such as heat exchangers, frequently requires the joining of complex subassemblies. These joints usually must be strong at high temperatures, vacuum or gas tight, and resistant to a variety of corrosive atmospheres. Moreover, although high reliability is required, cost is often an important factor.
These requirements usually exclude the cheaper methods of joining ceramics such as adhesives, cements, and pressed mechanical joints. Ceramic surfaces can also be joined by forming a heterogeneous joint such as by brazing with refractory metals or with ceramic brazes, such as glasses. Another kind of heterogeneous chemical bond is produced in the process disclosed in U.S. Pat. No. 4,050,956 by heating an abutting assembly of a ceramic oxide and a metal to a temperature below the melting point of any component of the assembly. However, because the brazing or bonding material has different properties than the ceramic itself, there are usually substantial limitations in service temperature, strength, and resistance to corrosive atmospheres. Welded joints, whether fabricated by solid-state diffusion or fusion, have superior high-temperature strength and corrosion resistance but are generally expensive and of limited application. Diffusion welding produces extremely strong, homogeneous bonded joints but vacuum tightness is difficult to obtain and unreliable. This technique is also slow, expensive, and somewhat limited with regard to the shapes that can be joined. Fusion welding can be accomplished by lasers, focused arcs, electron beams, and r.f. or gas torches, but in all cases the joint is shallow, limited to simple external configurations, and prone to thermal stress cracking. Therefore, none of the techniques currently used to join ceramics is completely satisfactory for forming gas-tight joints or joining complex surfaces at reasonable cost.